Explicitly construct URLSearchParams with string pairs

Although Chrome, Firefox, and Safari all seem to implement support for `new URLSearchParams(formData)`, the [spec][1] doesn’t mention it, the IDL and [TypeScript][2] don’t have typings for it, and searching suggests it’s a recent addition.

[1]: https://url.spec.whatwg.org/#dom-urlsearchparams-urlsearchparams
[2]: microsoft/TypeScript#30584

src/core/drive/form_submission.ts

@@ -52,8 +52,8 @@ export class FormSubmission {
   constructor(delegate: FormSubmissionDelegate, formElement: HTMLFormElement, submitter?: HTMLElement, mustRedirect = false) {
     this.delegate = delegate
     this.formElement = formElement
-    this.formData = buildFormData(formElement, submitter)
     this.submitter = submitter
+    this.formData = buildFormData(formElement, submitter)
     this.fetchRequest = new FetchRequest(this, this.method, this.location, this.body)
     this.mustRedirect = mustRedirect
   }
@@ -73,7 +73,7 @@ export class FormSubmission {
 
   get body() {
     if (this.enctype == FormEnctype.urlEncoded || this.method == FetchMethod.get) {
-      return new URLSearchParams(this.formData as any)
+      return new URLSearchParams(this.stringFormData)
     } else {
       return this.formData
     }
@@ -83,6 +83,12 @@ export class FormSubmission {
     return formEnctypeFromString(this.submitter?.getAttribute("formenctype") || this.formElement.enctype)
   }
 
+  get stringFormData() {
+    return [ ...this.formData ].reduce((entries, [ name, value ]) => {
+      return entries.concat(typeof value == "string" ? [[ name, value ]] : [])
+    }, [] as [string, string][])
+  }
+
   // The submission process
 
   async start() {